The present invention relates to a cut-off valve for power steering systems, in particular for automobiles, for limiting the pressure in at least one pressure chamber. The cut-off valve includes a piston that is adjustable against the force of a spring, and a fixed throttling point, which is arranged in a connecting line between a pressure chamber to which a first pressure is applied and a second pressure chamber to which a second pressure is applied.
A cut-off valve of this type is described in German Published Patent Application No. 42 16 053, in which it is possible to avoid response pressure tolerances of the cut-off valve almost completely. Moreover, manufacturing tolerances of the cut-off valve can be eliminated through this, and further tolerances of other steering components can be compensated for.
However, the conventional measures have no influence over the width of the tolerance field of the valve characteristics. When the conventional cut-off valve is applied, the hysteresis of the valve characteristics is too large for some applications.
It is an object of the invention to improve the cut-off valve so that the width of the tolerance field of the valve characteristics may be reduced to at least 50%.
The above and other beneficial objects of the present invention are achieved by providing a cut-off valve, in which the cross-section of the throttling point is made externally adjustable without dismantling. Through changing the cross-section of the throttling point it is possible to reduce the width of the tolerance field, or the width of the hysteresis of the valve characteristics respectively, so that it amounts to approximately half of the previous value.
The cross-section of the throttling point may either be infinitely adjustable or adjustable in multiple steps. Infinite adjustment may be effected very simply in that the throttling point is formed by a bore, the cross-section of which is, for example, adjustable by an adjusting screw or an adjusting member, the axis of which is substantially perpendicular to the axis of the bore. In this example embodiment, the cross-section of the bore is altered in that the adjusting screw is turned in to a greater or lesser depth.
Stepped adjustment of the cross-section of the throttling point may be effected by the following characteristics: The throttling point is formed by one of multiple throttle bores that are arranged on a rotatable, cylindrical adjusting member in one plane, spaced at a certain angular pitch, and which have varying cross-sections. One of the throttle bores acts in conjunction with a larger fixed bore in the housing, whereas the other throttle bores are closed. Finally, one of the throttle bores is arranged in the connecting line between the retroaction chamber and the return line to the container. For production reasons the throttle bores may be arranged in the rotatable adjusting member in one plane and at equal angular distances.